There have conventionally been known inkjet printers including an inkjet head having a plurality of channels for ink ejection. Such conventional inkjet printers are capable of controlling ink ejection while moving the inkjet head relatively with respect to a recording medium such as a sheet of paper or cloth when outputting a two-dimensional image onto the recording medium. Such ink ejection can be performed, for example, by using a pressure-type actuator (such as a piezoelectric, electrostatic, or a thermal-deformation actuator), or by thermally forming a bubble in ink in a tube. Among such pressure actuators, piezoelectric actuators are advantageous in, for example, that it is large in output, capable of being modulated, high in responsiveness, and adaptable to any type of ink, and thus piezoelectric actuators have been widely used in recent years.
There are two types of piezoelectric actuators, one using a bulky piezoelectric body made by sintering, such as a ceramic tile, the other using a thin-film piezoelectric body (a piezoelectric thin film) formed on a substrate. The former has a large output and thus is capable of ejecting ink droplets of a large size, but it is large in size and high in cost. In contrast, the latter has a small output and thus is not capable of ejecting ink droplets of a large size, but it is small in size and low in cost. Thus, it can be said that actuators configured with a piezoelectric thin film are suitable to realize high-resolution (which can be achieved with small ink droplets) low-cost printers. Whether to use a piezoelectric thin film or a bulky piezoelectric body in a piezoelectric actuator may be decided in accordance with the usage of the actuator.
FIG. 15 shows a plan view illustrating a general configuration of an inkjet head 200 including a conventional piezoelectric actuator 101, and a sectional view of the inkjet head 200 taken along line A-A′ in the plan view. The inkjet head 200 is configured with a head substrate 100 having a pressure chamber 100a, an actuator 101 arranged on one side of the head substrate 100, and a nozzle substrate 102 arranged on the other side of the head substrate 100. In the nozzle substrate 102, there is formed a nozzle hole 102a for controlling an amount of ink droplets. The nozzle hole 102a communicates with the pressure chamber 100a. 
The actuator 101 is configured with a diaphragm (a driven film) 201, an insulating layer 202, a lower electrode 203, a piezoelectric body layer 204, and an upper electrode 205, laid one on top of another in this order from the head substrate 100 side. The lower electrode 203 and the upper electrode 205 are connected to a driver circuit 206. Furthermore, an ink feeding hole 301 for feeding ink from an unillustrated storage chamber into the pressure chamber 100a is formed through the diaphragm 201 and the insulating layer 202. The ink feeding hole 301 communicates with the pressure chamber 100a via a sub-chamber 100b formed beside the pressure chamber 100a in the head substrate 100.
In the above configuration, when voltage is applied to the lower electrode 203 and the upper electrode 205 from the driver circuit 206, the piezoelectric body layer 204 expands/contracts in a direction perpendicular to its thickness direction (that is, a direction parallel to the surface of the head substrate 100). The piezoelectric body layer 204 and the diaphragm 201 have different lengths, and this difference in length generates a curvature in the diaphragm 201, and as a result, the diaphragm 201 is displaced (curved) in its thickness direction. This up-and-down movement of the actuator 101 applies pressure to the ink introduced into the pressure chamber 100a, and thereby ink droplets can be ejected through the nozzle hole 102a. 
This use of the actuator 101 in combination with the head substrate 100 and the nozzle substrate 102 makes it possible to form an ink channel (an ink ejection portion), and the inkjet head 200 is constituted by arranging such ink channels in length and width directions.
There are cases where foreign objects, such as waste generated during processing and assembly, may adhere to an inside of the inkjet head. There are also cases where the supplied ink includes foreign objects such as waste and flocculated particles. Furthermore, since the pressure chamber is in a negative pressure state after the ink is pushed out therefrom and ejected through the nozzle hole, air bubbles may be formed in the ink due to cavitation. With such foreign objects or air bubbles formed in an inside of the pressure chamber, the nozzle will be clogged up or pressure will be lost, and as a result, it becomes impossible to eject ink.
In order to remove such foreign objects and air bubbles from the inside of the pressure chamber, it is necessary to make ink in the pressure chamber circulate. In this regard, in Patent Literature 1, for example, a control mechanism (which includes a pump, a flow path, and a controller) is provided outside a head, and this control mechanism makes ink continuously circulate outside and inside the head.